Internal combustion engines emit a large amount of unburned hydrocarbons. The hydrocarbons include low molecular weight alkanes and alkenes. Of these, the low molecular weight alkenes (olefins) are very troublesome because they have a great propensity to form ozone in the atmosphere and are most difficult to trap by adsorption. Ozone is a major smog forming constituent. For this reason, emission standards are becoming more stringent with regard to ozone-forming hydrocarbons.
Internal combustion engines emit a large amount of unburned hydrocarbons during cold engine start-up. In fact, a substantial fraction of the total emitted hydrocarbons have been found to occur during the first ten seconds due to the rich fuel mixture.
Release of hydrocarbons after starting an engine poses a special problem because at this point the temperature is not high enough for conversion to innocuous products in the presence of conventional catalysts. The catalysts utilized in catalytic converter systems are generally ineffective at ambient temperature and must reach high temperatures, often in the range of 300.degree.-400.degree. C. before they are activated.
One method of reducing cold start emissions is to temporarily adsorb hydrocarbons on zeolites. One disadvantage of this method is that exhaust gas contains water which strongly inhibits the adsorption capacity of some zeolites, especially for olefins.
Therefore it would be desirable to have a hydrocarbon removal system in which water does not affect the adsorption of hydrocarbons.